A reconfigurable optical add/drop multiplexing (ROADM) device is one conventional example of a device that subjects wavelength-multiplexed light from a plurality of different paths to wavelength division or wavelength multiplexing and transmits the resulting light. Technology has been disclosed in which, in this kind of device, if it is determined that wavelength-multiplexed light from a certain path is in an input-interrupted state, the output of a transmission-purpose optical amplifier is maintained at a fixed level by emitting dummy light and switching the signal light that has been determined as being in an input-interrupted state with the dummy light and performing transmission. This technology is disclosed in Japanese Laid-open Patent Publication No. 2012-109653, for example.